When driving a motor that has electrical saliency, such as a synchronous motor, to produce a required torque, it is necessary to detect or infer a rotor position and supply an appropriate current according to a position of a magnetic pole of the motor.
To infer a rotor position of a motor having electrical saliency, it is known to apply a current or voltage that has higher frequency than a power source which generates a torque in a rotor. With such an application, a high frequency wave is superposed (also referred to as “superpositioned”) on a wave of the power source. Hereinafter, the power source wave, either current or voltage, is referred to as a “basic wave”, and its frequency is referred to as a “basic frequency”. In this conventional method, the rotor position can be inferred by analyzing the component of the high frequency wave that is applied to the basic wave. According to this conventional method of inferring the rotor position, a speed of a motor can be controlled precisely because a current having an appropriate phase is supplied to the motor. To obtain sufficient current or voltage to infer the rotor position, the high frequency wave applied to the basic wave must have at least a particular amplitude. Hereinafter, this conventional method of control is referred to as a “high frequency wave superposition sensorless control.” In the high frequency wave superposition sensorless control, a method that applies a high frequency wave on either a d-axis or a q-axis, of a rotary coordinates system of dq axes for vector control, may be referred to as a “single superposition.” A method that applies a high frequency wave on both a d-axis and q-axis may be referred to as a “double superposition.”
In this conventional method, the higher frequency wave tends to cause an electrical noise that is offensive to human ears.